The influence of temperature, bulk composition, and melting on the seismic signature of the low-velocity layer above the transition zone

We report a new technique to describe seismic velocity and impedance anomalies atop a seismic low‐velocity layer (LVL) at 350 km depth. We model shear wave speed reductions detected with Ps conversions beneath the Hawaiian Islands and negative impedance contrasts detected with ScS reverberations ben...

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Bibliographic Details
Published in:Journal of geophysical research. Solid earth 2014-02, Vol.119 (2), p.971-983
Main Authors: Hier-Majumder, Saswata, Keel, Ellen B., Courtier, Anna M.
Format: Article
Language:English
Subjects:
Basalt
Conductivity
Corals
Dihedral angle
Geophysics
High temperature
Impedance
Islands
LVL
magma dynamics
Mathematical models
melting
Melts
microgeodynamics
MuMaP
Seismic properties
Seismology
Signatures
Temperature
Transition zone
Velocity
Volume fraction
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Summary:We report a new technique to describe seismic velocity and impedance anomalies atop a seismic low‐velocity layer (LVL) at 350 km depth. We model shear wave speed reductions detected with Ps conversions beneath the Hawaiian Islands and negative impedance contrasts detected with ScS reverberations beneath the Coral Sea in the South Pacific, by varying the bulk solid composition, reference potential temperature, dihedral angle of melt, and melt composition. For a given bulk solid composition, the effects of elevated temperature and melt volume fraction on the seismic properties trade off with one another. At a given temperature, the calculated melt volume fraction is nearly insensitive to variations in the bulk solid composition. A low volume fraction of low dihedral angle melts mimics the seismic signature of a higher volume fraction of high dihedral angle melts. Despite stronger lateral variations in the LVL structure beneath Hawaii compared to the Coral Sea, both regional averages are similar. For a basalt volume fraction of 0.2 and a dihedral angle of 10°, we estimate regional averages of 1.1 ± 0.8 vol % melt at a depth of 350 km beneath the Hawaiian Islands for a reference potential temperature of 1800 K and 1.2 ± 0.005 vol % melt at a depth of 350 km beneath the Coral Sea region for a reference potential temperature of 1500 K. Our model of the seismic signal is unable to distinguish between melt compositions of mid‐ocean ridge basalt and carbonated peridotite melts at such small melt volume fractions. Key Points One percent melt in the LVL above transition zone Same melt volume fraction in subduction and plume settings Electrical conductivity better matches observations with silicate melts
ISSN:2169-9313
2169-9356
DOI:10.1002/2013JB010314